News and Information
18th June 2018
No rain but the village still had water
When the Hydro was in the design stage a lot of time and effort was spent looking at how to interface the Village Water supply with the Hydro Scheme in a way that would protect both the integrity of the Hydro and the reliability of the water supply for years to come.
The chosen solution was to have two separate pipes running out of the intake chamber, the base of the much larger hydro pipe slightly higher up the side wall of the chamber than the base of the smaller water pipe. The Hydro pipe is huge at 1200mm internal diameter whilst the water supply pipe is 160mm diameter, which is significantly larger than the old 90mm diameter pipe. Both pipes run in the same trench until the top of the hill above the Power House where the water pipe diverts across to the water treatment plant and the Hydro pipe heads down the hill.
The other improvement was to add a scour pipe to the intake to ensure that the bottom of the tank is kept as clean as possible. The Hydro Operations team routinely (once a week) open the valve to a run off pipe that allows water to run out of the bottom of the Intake chamber and back in to the river about 50 meters downstream of the intake. This allows the movement of the water to clean out the base of the tank. The operator can, by checking the quality of the water coming out of the run off pipe, assess the state of the base of the tank.
The intake tank is kept full by water flowing over the bar screen that runs the length of the chamber on the river side of the intake. The river water level is protected by two means:
- There is a compensation slot in the wall that connects the intake to the old weir, this ensures that a certain level of water will always go downstream
- The bar screen is higher on the river side than on the bank side,
this does two things:
- It ensures that most of the river borne debris is washed over the bar screen into the run-off channel, keeping the bar screen clear
- If the river level falls too low, no water enters the intake and it all goes down river.
This solution complied with SEPA’s (Scottish Environmental Protection Agency) requirements but it had one fundamental flaw: What would happen in a prolonged period of low or zero rainfall? The community would be constantly drawing water from an intake chamber that was not being replenished. The intake chamber is large and holds enough water to keep the water supply tanks full for many days, even weeks depending on demand but the team still felt that the risk was too high, as there was a small possibility that the village could run out of water. When pushed the experts admitted that a low probability was a less than 5% chance. Obviously, those of us who lived in the village felt that less than 5% was way too high! So a solution was commissioned and found.
That solution was to put a special intake valve on the river side of the bar screen that would take in enough water to maintain the village water supply unless the river dried up, which is highly unlikely. The valve is a special elbow jointed valve that can be turned 90o to allow it to be cleaned, it sits virtually on the river bed, alongside the bar screen, and in front of the compensation slot, meaning that it takes priority over everything else. It also means that water in the intake tank is constantly replenished at a rate that should keep pace with demand.
The pictures above were taken on 8th June 2018; at the top in the first picture,
you can see the old Victorian weir, showing just how low the river was,
at the bottom you can see the edge of the bar screen which is completely dry,
and the green and silver elbow valve is sitting in the river.
It is, as can be seen in the next photograph, still completely under water. The bar screen, however, is high and dry!
The Hydro stopped generating on 26th May, meaning that the Hydro was drawing no water at all, leaving the full capacity of the tank, available for the local water supply.
On the 13th June, we had first hand proof that the elbow valve was doing
its work. There is a
Head Level Sensor in the intake chamber, which
records how much water is above the sensor. The senor is located about
5 meters above the floor of the tank. We also have a river level sensor,
situated on the exterior of the intake at the upstream end. Those two
sensors allow us to monitor turbine performance and keep a track of the
The turbine will not run if it cannot maintain a head level above 0.5m, nor will it run if it falls below 10% of its generating capacity. It will not restart until the head level is reading 1.22m. On the 26th May, when the turbine stopped, the river level read 1.197m and the Head level was 0.979m. By the 8th June, when these photos were taken, the river levels were too low to allow water over the bar screen.
On the 11th June, the river level was 1.091m and the head 1.049m meaning that the elbow valve was keeping ahead of the local water demand. At its worst, on 13th June at 15:06 the river level was 1.051m and the head level 1.026m.
The river level was 0.146m lower than when the turbine stopped but the head level was still 0.047m higher.
The elbow valve did its job, water supplies were maintained and nobody in the village noticed anything.
The Hydro team are delighted that all the planning, thinking, and desktop scenarios, resulted in a solution which has not only future-proofed the community water supply but which has ensured that the water will keep flowing even in periods of dry weather.